Echoes from the future? A physicist speaks

On a high balcony in Laguna Beach that gives new meaning to the phrase, "sun-drenched," National Medal of Science winner Yakir Aharonov was warming up for a public talk this Wednesday on one of the strangest results in physics: that the future, at least among atomic particles, can subtly influence the past.

It might be the opening lines of the latest science-fiction novel -- except that Aharonov's theory has been verified in laboratory experiments, and appears to be valid.

But the Israeli physicist often can be found at the high perch of the house in Laguna, rented by Chapman associate physics professor Jeff Tollaksen and nicknamed the "quantum house."

They and other physicists sometimes gather there to work out equations on a white board, debating and occasionally arguing over the nature of reality.

Aharonov's findings seem to challenge conventional notions of time and even free will.

He is best known as the co-discoverer, in 1959, of the Aharonov-Bohm effect, when a charged particle is influenced by a magnetic field with which it has never come into contact.

It's an example of a weird property in quantum mechanics -- the physics of the very small -- known as "non locality."

But some of the planned talk, at 5 p.m. in Chapman's Irvine Lecture Hall, will focus on one of his other strange results: that a long series of gentle or "weak" measurements can reveal information many physicists consider off limits, forbidden by the nature of physical reality.

These weak measurements, Aharonov said, suggest that the present is a kind of collision between information from the past and from the future.

And it could well explain another bizarre effect in quantum physics. Subatomic particles can appear to be in two or more places at once; in reality, Aharonov said, a given property of the particle, say, its spin, might be separated from the particle itself.

He calls it it the quantum Cheshire cat effect, a reference both to Lewis Carroll and to a famous physicist, Erwin Schrodinger.

"It describes a cat who gets angry, and decides to disappear," Aharonov explained this week to visitors at the quantum house. "He does it in a very funny way. The tail disappears, then the face disappears -- until finally, there's just a smile. It turns out that this does happen in quantum mechanics."

In the early days of the field, Schrodinger devised a thought experiment to highlight the strange implications of a particle being in more than one location at once, known as a "superposition" of states.

In the thought experiment, a cat lies in a sealed chamber with a vial of poison nearby. The decay of a quantum particle, random and unpredictable, can break the vial, release the poison and kill the cat.

Without opening the chamber, however, there is no way to know whether the particle has decayed, and whether the cat is alive or dead.

The cat is said to exist simultaneously in a superposition of both states: alive and dead, at least until the chamber is opened.

Aharonov updates the story, and renders it a bit more humane, by turning the vial of poison into a sleeping pill in a bowl of milk.

In his story, Schrodinger's cat is both asleep and awake.

"The cat will drink the milk and die, if you want to be cruel, or fall asleep if you want to be gentle," Aharonov said.

But using weak measurements, we might learn that a Cheshire cat, the only kind capable of smiling, can be found in two different chambers.

Most of it, minus the smile, is in one room. The smile itself is in the other.

In other words, a property of the particle is separated from the particle itself, explaining how it appears to be in two places at once -- a profound alteration in physicists' view of time and space.

But the implications of weak measurements can be even more startling.

Quantum physics includes the "uncertainty principle," which says our efforts to measure different aspects of a particle have a built-in limit.

The particle must be disturbed -- say with a beam of light -- to make a measurement. The more precise the measurement, however, the more the disturbance interferes with our ability to measure something else.

Measure the speed of the particle precisely and your measurement of its position becomes fuzzy. Or, pin down its position precisely, and you can't tell how fast it's moving.

But Aharonov reinterprets this limit. By conducting extremely weak measurements that reveal only tiny amounts of information with each repetition, he can, after a long series of such measurements, bring the disturbance factor down to zero.

And by adding up the various bits of information, he can gain a clear picture otherwise unobtainable.

"What I discovered was very surprising," Aharonov said. "In fact, if you're willing to look at the particle and get very little information, and repeat the experiment many, many times, you can collect all this information, then find out something about the state of each particle."

In the laboratory, that has allowed scientists to reveal the influence of the future on the past, at least in the microscopic realm, he says.

A mass of information collected in the present might appear disorganized and meaningless, a jumble of noisy signals.

But by making certain measurements in the future, some of the separate bits of noisy information are suddenly revealed as part of a coherent whole.

It's as if a message, "the cat is smiling," is broken up and scattered among a series of sealed envelopes, then mixed in with many other envelopes containing random noise.

Only measurements in the future can reveal the key that allows the message-bearing envelopes to be found and reassembled to give meaning; otherwise, they are just part of the noise.

"The difference between the particles cannot be discovered in the past," he said. "Maybe nature is trying to tell us that the world, in the present, should be described by two informations: one coming from the past, and one coming from the future."

In other words, the future influences and alters the past.

"This is basically a new reformulation of physics," Aharonov says. "It looks at time in a new way."

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